Programs with Quasi-stable Channels Are Eeectively Recognizable

We consider the analysis of in nite half-duplex systems which consists of nite state machines that communicate over unbounded channels. The property half-duplex for two machines and two channels (one in each direction) says that each reachable state has at least one channel empty. The contributions of this paper are (a) to give a nite description of the reachability set of such systems, which happens to be e ectively recognizable; this description allows us to solve classical veri cation problems such as: whether a given state is reachable, whether there exist deadlock states, whether the reachability set is nite and whether a speci ed action is useless; (b) to propose an extension of these results for a new class, systems with quasi-stable channels, which includes systems with similar behavior but which implies more than two machines.